This invention relates to vapor-phase curable coating compositions employing aliphatic polyisocyanates.
In the field of industrial coatings, particularly in the automotive industry, the speed and ease of application and cure are extremely important. It is for this reason that paint manufacturers have sought an industrial coating system that can be easily applied and which is capable of an easy and rapid cure to a tack-free state. One recent attempt at a workable system is the use of a vapor-phase catalyst to cure a coating composition. In the typical practice of such a system, a substrate is coated with a composition containing an aromatic polyisocyanate and a polyhydroxyl compound (polyol), and the coated substrate is then exposed to the vapor of a "urethane" catalyst, typically a tertiary amine.
Although these vapor-phase curable coating compositions are easy to apply and easy to cure, they have a serious shortcoming. Their practice requires the use of an aromatic polyisocyanate, because the use of an aliphatic isocyanate will increase the cure time to several hours. Unfortunately, however, aromatic polyisocyanates, unlike their aliphatic counterparts, cause yellowing in the ultimate cured finish when exposed to ultraviolet light. Thus, they are totally unacceptable for use in applications that cannot tolerate changes in color after cure.
Considering specific prior art, U.S. Pat. No. 3,874,898 is relevant in that it teaches vehicles, including primarily printing inks, but also including surface coatings, containing a polyisocyanate and a polyol, which is cured by use of an amine vapor.
U.S. Pat. No. 4,343,924 and four related patents, U.S. Pat. Nos. 3,822,226; 3,789,044; 3,836,491; and GB No. 1,351,881 (all five of which are assigned to Ashland Oil) all relate to coatings containing an isocyanate and a polyol, which are cured by a vapor catalyst. Although some of these four patents mention that the use of functionally substituted amines such as dimethyl ethanolamine (a mono-ol) is not outside their claim scope, they also teach that tertiary amines containing active hydrogen are less preferred because of their potential reaction with the isocyanate. Further, U.S. Pat. No. 4,343,924 explicitly states that aromatic isocyanates are necessary in order to obtain a desired rapid reaction in the presence of the vaporous tertiary amine catalysts at room temperature.
U.S. Pat. Nos. 3,892,713 and 4,143,009 both discuss isocyanate systems having a tertiary amine polyol, but these patents do not involve vapor phase curing.
BE Nos. 876,906 and 876,907 both teach foundary binders which employ an aromatic amine polyol, and which are cured by a vapor catalyst. However, these compositions are intended to be easily broken into fragments, and therefore would not be suitable as coating compositions. The above-mentioned coating patents to Ashland Oil also teach that foundary binders are not generally suitable for use in coating compositions. Further, these two foundary binder patents teach a preference for aromatic isocyanates.
Therefore, it would be desirable to have a vapor-phase curable urethane coating composition employing an aliphatic isocyanate, which could cure generally as fast as an aromatic isocyanate system, but which would not suffer from the yellowing inherent in such systems.